Recently, interest in energy storage technologies has increased. As the energy storage technologies are extended to such devices as cellular phones, camcorders and notebook PC, and further to electric vehicles, endeavors for research and development of electrochemical devices have become more focused. Therefore, the electrochemical devices have been drawing an attention in this aspect, and among them, the interest has focused on the developments of rechargeable secondary batteries.
Among the currently used secondary batteries, a lithium secondary battery developed in the early 1990's allows repeated charging/discharging as lithium ions are intercalated into or disintercalated from cathode and anode. This lithium secondary battery may convert chemical energy into electrical energy by means of oxidation and reduction reactions. Since the lithium secondary battery generally has an average discharge voltage of about 3.6V to about 3.7V, it is in the spotlight from the viewpoint of higher operation voltage and greater energy density in comparison to conventional batteries such as Ni—MH or Ni—Cd batteries.
The lithium secondary battery may include a cathode, an anode, a porous separator and an electrolyte. The electrolyte is generally made using a carbonate-based organic solvent such as ethylene carbonate (EC) and dimethyl carbonate (DMC) as an electrolyte solvent and a lithium salt such as LiPF6 and LiBF4 as an electrolyte salt. In order for the battery to have such a higher operation voltage as mentioned above, an electrolyte composition should be electrochemically stable in a charging/discharging voltage range from about 0 to about 4.2V.
However, a carbonate-based organic solvent is generally decomposed on the surface of an electrode during the charging/discharging process, so it may cause side reactions in the battery. For example, an electrolyte solvent having a large molecular weight such as ethylene carbonate (EC), dimethyl carbonate (DMC) or diethyl carbonate (DEC) is cointercalated between graphite layers in a carbon-based anode. This cointercalation may destroy the anode structure. As a result, the performance of a lithium secondary battery may deteriorate gradually as charging/discharging process is repeated.
It is known in the art that the above problems may be solved using a solid electrolyte interface (SEI) film formed on the surface of an anode by a reduction reaction of a carbonate-based organic solvent during the initial charging process. However, the SEI film is generally insufficient in continuously acting in the role of a protective film for an anode. In addition, as the reduction reaction on the surface of the anode is repeated, the battery capacity may be reduced and the battery life cycle may be shortened. Further, while the SEI film is formed, the carbonate-based organic solvent may decompose, which may generate gas such as CO, CO2, CH4, C2H6, etc.
Furthermore, the SEI film is thermally unstable. Thus, when a battery is left at a high temperature in a fully charged state, the SEI film may be easily broken down due to increased electrochemical energy and thermal energy over time. It induces continuous side reactions between the surface of the anode and the electrolyte, and decomposition of the electrolyte, and may continuously generate gas such as CO2. Accordingly, the inner pressure of the battery may be increased, thereby increasing the thickness of the battery. This may cause performance problems in electronics such as cellular phones and notebook computers with regard to high temperature performance of the battery.
In order to solve the above problems, there have been suggested a method of adding a sulfide-based compound to an electrolyte to restrain decomposition of the electrolyte and a method of adding diphenyl picrylhydrazyl (DPPH) to improve high-temperature stability. However, when the above specific compounds are added to an electrolyte to improve the battery performance, some areas of performance are improved, but other areas of performance may deteriorate. Likewise, in many conventional batteries, only certain areas of performance are improved.
Meanwhile, an electrolyte solvent of a lithium secondary battery generally employs ethylene carbonate which is a cyclic carbonate compound. However, since ethylene carbonate has a high freezing point (37 to 39° C.), a battery using the ethylene carbonate may exhibit a poor low temperature performance. To solve this problem, Japanese Laid-open Patent Publication No. H07-153486 discloses a lithium secondary battery using an electrolyte made by adding 0.5 to 50 volume % of γ-butyrolactone to a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate. However, if γ-butyrolactone is added in this manner, the life cycle of the battery may be shortened though the high-rate discharge characteristic at a low temperature is improved.
Japanese Laid-open Patent Publication No. H06-20721 discloses a secondary battery using a non-aqueous electrolyte to provide a high-capacity secondary battery, which includes an anode of carbon material containing graphite with its plane interlayer spacing (d002) being less than 0.337 and a non-aqueous electrolyte solvent containing 20 to 50 volume % of γ-butyrolactone and the remaining volume % of a cyclic carbonate. However, since the above non-aqueous solvent does not include a straight-chain carbonate, this electrolyte solvent has a high viscosity and low ionic conductivity and the battery prepared therefrom exhibits deteriorated low temperature discharge capacity.
In addition, a method has been proposed for improving the charging/discharging characteristics of a battery at room temperature and a low temperature by employing a linear ester compound having a low viscosity as an electrolyte additive/solvent (see Japanese Laid-open Patent Publication Nos. H05-182689 and H04-284374). However, the employed linear ester compound has a high reactivity with the graphite anode frequently used in secondary batteries such that it may cause side reactions in the battery or deteriorate other performances of the battery.
This problem occurs especially when the anode has a large specific surface area. The larger the specific surface area of the anode, the more the linear ester compound reacts with the anode active material. Consequently, an excessive reduction reaction of the anode is induced. This side reaction proceeds more rapidly at a high temperature, resulting in a decline in the battery performances.
Thus, there is a demand for developing a lithium battery capable of providing a sufficient charging/discharging life cycle, effective high-temperature stability and effective low temperature discharging characteristics by changing the composition of a non-aqueous mixture solvent used in an electrolyte of a conventional lithium battery.